This commit updates the CLI so that it will decrypt any encrypted
passwords in a user's Maven settings.xml file.

The code that performs the decrytion has a transitive dependency on
three types in Plexus' logging API. There are tens of different
artifacts containing this API available in Maven Central. Rather than
bloating the API with a dependency on a complete Plexus container,
which could perhaps be considered the primary source, a dependency on
a considerably smaller artifact has been introduced.

Closes #574